The purpose of this study is to evaluate perfusion density of an old myocardial infarction using cardiac multidetector computed tomography (MDCT) with arterial phase. We included 56 patients with an old myocardial infarction who had undergone medical treatment, primary angioplasty or bypass graft. Cardiac MDCT without stress perfusion was used for calculation of density of myocardial perfusion and global left ventricular function in patients was also analyzed according to initial and last follow-up echocardiography. All patients showed decreased perfusion in the left ventricular wall. Densities of normal ventricular wall, cavity of the left ventricle, and infracted wall were 105±29 HU, 471±117 HU, and 11±29 HU (-64 HU-57 HU), respectively. Subendocardial fat depositions were observed in 19 patients (34%). Ejection fractions of initial and last follow-up periods near to the date of cardiac CT were 61±9 % and 57±12 %, respectively. In conclusion, decreased perfusion less than 60 HU in cardiac CT with arterial phase was helpful in diagnosus of an old myocardial infarction.